1. Field of the Invention
The present invention relates to a process for production of 2,6-dimethylnaphthalene. More particularly, it pertains to a process for efficiently producing 2,6-dimethylnaphthalene useful as a starting raw material for 2,6-naphthalene dicarboxylic acid by the use of a specific catalyst.
2. Description of Related Art
Polyethylene naphthalate which is obtained by the condensation reaction of 2,6-naphthalene dicarboxylic acid with ethylene glycol is excellent in tensile strength and heat resistance and is widely used for fibers and films.
2,6-Naphthalene dicarboxylic acid which is subjected to condensation reaction in the production of polyethylene naphthalate is generally derived from 2,6-dimethylnaphthalene, which has an industrially important use as a starting raw material for the production of such high-performance polyester.
2,6-Dimethylnaphthalene has heretofore been obtained by isolating it from a coal tar fraction or a fraction of heavy oil subjected to fluid catalytic cracking (FCC). However, the aforementioned isolation process affords the fraction in the form of mixture containing almost all the types of methyl group-position isomers such as 1,2-, 1,3-, 1,4-, 1,5-, 1,6-, 1,7-, 1,8-, 2,3- and 2,7-dimethylnaphthalene in addition to the objective 2,6-dimethylnaphthalene. Thus, the process for isolating and purifying 2,6-dimethylnaphthalene from such fraction mixture suffers the disadvantage of a number of steps and high cost required in the production thereof, making itself unsuitable for inexpensive mass-production of 2,6-dimethylnaphthalene.
There have recently been proposed several processes for producing 2,6-dimethylnaphthalene from a variety of starting raw materials, but there has not yet been established an industrial production process capable of efficiently and selectively synthesizing 2,6-dimethylnaphthalene by the use of inexpensive starting raw material available in a large quantity.
Examples of the above-mentioned process include the process disclosed in Japanese Patent Application Laid-Open Nos. 172937/1985, 29536/1987 and 8344/1988 in which naphthalene or monomethylnaphthalene is methylated into dimethylnaphthalene and the process disclosed in Japanese Patent Application Laid-Open Nos. 45536/1985 and 14737/1988 in which naphthalene or monomethylnaphthalene is transmethylated into dimethylnaphthalene by using polymethylbenzene.
Nevertheless, the process disclosed in any of the aforesaid Laid-Open Patent Applications suffers from the defects that the conversion of naphthalene and monomethylnaphthalene is low and selective production of 2,6-dimethylnaphthalene is difficult. Consequently, the process necessitates complicated isolating purification steps and intricate isomerization steps, thus causing disadvantageous problem from the industrial point of view.
In addition, Japanese Patent Application Laid-Open Nos. 61647/1973 and 48647/1974 disclose the process for production of 2,6-dimethylnaphthalene by cyclization, dehydrogenation and isomerization of 5-(o-tolyl)pentene-2 to be used as a starting material. Moreover, Japanese Patent Publication Nos. 17983/1975, 17985/1975 and 22550/1975 disclose the process for producing dimethylnaphthalene by cyclization dehydrogenation of 5-(o-tolyl)pentene-2.
In the above-mentioned processes, however, 5-(o-tolyl)pentene-2 to be used as a starting material is usually produced from o-xylene and 1,3-butadiene by the use of an alkali metal such as potassium or sodium as a catalyst, and many problems remain unsolved with regard to the catalyst handling especially in safety. Likewise, as the resultant dimethylnaphthalene is obtained as the mixture of isomers such as 1,5-dimethylnaphthalene, 1,6-dimethylnaphthalene, 2,6-dimethylnaphthalene and the like, the production of 2,6-dimethylnaphthalene therefrom suffers a lot of disadvantages that the steps of isomerization, separation and purification are necessary.
On the other hand, there has been developed a process for selectively producing 2,6-dimethylnaphthalene by cyclization and dehydrogenation of 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)butane as disclosed, for example, in Japanese Patent Publication No. 5292/1978 wherein a catalyst comprising rhenium oxide, an alkali metal oxide or alkaline earth metal oxide and alumina is used, and in Japanese Patent Publication No. 1701/1976 wherein a catalyst of chromia/alumina series containing an alkali metal oxide is employed.
However, the above-disclosed catalysts are unfavorable, since the use thereof results in a low yield and an insufficient purity of the objective 2,6-dimethylnaphthalene; besides the highly toxic chrominum compound therein will bring about environmental pollution problem.
It has been discovered by the present inventors, as disclosed in Japanese Patent Application Laid-Open Nos. 173834/1991 and 251545/1991, that in the production of 2,6-dimethylnaphthalene by cyclization dehydrogenation of 2-methyl-1-(p-tolyl)butene or 2-methyl-1-(p-tolyl)butane, 2,6-dimethylnaphthalene with high purity is obtained at a relatively high yield by the application of the catalyst comprising lead and alumina, or indium and alumina, as essential components.
Nevertheless, the above-developed catalysts still involve some problems such that the catalyst containing a lead component is industrially unfavorable due to its toxicity and the catalyst containing an indium component sometimes evaporates away during its use owing to the high volatility of a monovalent indium compound formed during the reaction and the like. Under such circumstances, it has been sought for a long time to develop a catalyst capable of producing highly pure 2,6-dimethylnaphthalene at a high yield with stabilized operation without causing any problem relating to sanitation.
In view of the above, intensive research and investigation were concentrated by the present inventors on the process for cyclization dehydrogenation of 2-methyl-1-(p-tolyl) butene, 2-methyl-1-(p-tolyl)butane and mixture thereof. As a result, it has been discovered by the present inventors that highly pure 2,6-dimethylnaphthalene is obtained at a high yield by the use of a catalyst comprising a platinum component and at least one compound selected from alkali metal compounds and alkaline earth metal compounds each being supported on aluminum oxide. The present invention has been accomplished on the basis of the above-mentioned finding and information.